Automated Personal Rapid Transit (PRT), where automobile sized (or smaller) vehicles independently operate on fixed rails. Most PRT system designs use passive rails (analogous to freeways) that rely on steering equipment in the vehicle to affect divergence to an alternative branch (analogous to an exit on a freeway).
The field of magnetically levitated vehicle suspension (maglev) includes a variety of technologies in the market. Most maglev vehicle suspension technologies have been optimized for use in trains; several rely upon active (mechanically movable) switch elements.
Train-type mechanical switch elements are limited in quickness, reliability and cost. Some mechanical switch elements wear over time, and reliability is dependent on the level of maintenance which can be extensive and costly. Importantly, mechanical switch elements take time to engage, with some taking more than a second. Mechanical switch elements are inappropriate where the frequency of vehicle traffic is designed having a capacity of one vehicle per ten second interval or less, and especially those with intervals of less than one second. Also most mechanical switch elements wear considerably when operating every few minutes, instead of every few hours as with many train-based systems.
Electromagnetic Suspension (EMS) such as U.S. Pat. No. 8,234,981 B2 Aug. 2012 Zheng et al; and U.S. Pat. No. 8,171,859 B2 May 2012 Loser et al both of Germany (Transrapid) uses feedback control of electromagnets in the vehicle interacting with soft magnetic elements to produce attractive levitation forces. EMS is able to maintain levitation without forward movement; however it requires constant supply of electric energy to produce the levitation force. EMS requires constant computer control to artificially stabilize inherently unstable magnetic forces. EMS trains have some magnetic drag. The sum of the drag energy and maglev operating energy is typically greater than the rolling resistance energy of the best steel wheel trains. Most EMS systems have been applied to train technology, and require the use of mechanical switch elements in the that seriously limit vehicle frequency.
Several maglev technologies use Neodymium permanent magnets (NdPM) in attraction, shear, or repulsion. NdPM can be made low drag if designed to minimize eddy current drag. Since NdPM maglev is inherently unstable, it must be stabilized by other forces such as rolling elements or EDS stabilization. Some examples are: U.S. Pat. No. 7,624,686 B2 December 2009, U.S. Pat. No. 7,380,508 B2 June 2008, U.S. Pat. No. 7,314,008 B2 January 2008, U.S. Pat. No. 7,484,462 February 2009, U.S. Pat. No. 7,484,463 B2 February 2009, U.S. Pat. No. 7,243,604 B2 July 2007, and Chinese Patent Publication No. CN1264660A titled “tube vacuum permanent magnetic compensation type levitation train-elevated railway-station system” all by Lingqun Li of Dalian China; and also US 2006/0236890 and U.S. Pat. No. 7,204,192 B2 April 2007 to Lamb et al (MagnaForce/Levex) of the US that uses NdPM in vehicle and where guidance force must stabilized with contacting rollers that are subject to speed limitations, wear and sudden failure. U.S. Pat. No. 5,218,257 June 1993 Tozoni of MD USA, and U.S. Pat. No. 5,225,728 July 1993 Oshima of Japan both claim stable PM maglev, however the configurations indicated have not been demonstrated to work even though now expired and in the public domain.
U.S. Pat. No. 7,757,609, issued July 2010, U.S. Pat. No. 6,684,794, issued February 2004, U.S. Pat. No. 6,374,746, issued April 2002 all to Fisk et al. uses NdPM in both the vehicle and to supply repulsive lift force, and EMS control to provide “steering” forces and stability, and the capability of full speed interchanges with converge and diverge frequency limited only by the vehicle length and speed and constant electronic measurement and control. Magtube relies on constant computer control for operation, as well as constant electrical supply to energize the electromagnetic coils. However, a constant electrical supply results in losses in electrical efficiency.
U.S. Pat. No. 7,448,327 B2 November 2008 Thornton et al. Assigned to Magnemotion (M3) uses NdPM to provide about half of the attractive levitation force, this reduces the energy requirements of EMS, but still relies on constant computer control and electric energy supply. M3 can levitate only up to ten times the magnet weight; and requires mechanical switching that limits vehicle frequency.
There is risk of loss of electric energy supply or computing power or stability augmentation with the above maglev systems; such failure would result in vehicle damage. Most of the systems have crash pads made of heat resistant friction material to absorb the kinetic energy of the vehicle in the event of a crash.
Electrodynamic Suspension (EDS) maglev can be configured so that mechanical switches are not required to inject vehicles into the flow of traffic. EDS maglev systems can be designed to produce stable levitation without constant electric supply or constant computer control. EDS systems require forward motion of the vehicle to produce levitation. EDS maglev uses magnetic fields in the vehicle to induce electric currents. The currents in the produce an opposing magnetic field that lifts the weight of the vehicle when velocity is sufficient. An example of EDS is the Japanese National Railway maglev train that currently holds the world record maglev speed. The Japanese EDS uses superconducting magnets in the vehicle interacting with an aluminum plate. The superconducting magnets must be cooled to liquid helium temperature so it is very expensive to operate. The magnetic drag of EDS is very high at low speed, reaching a maximum at “liftoff”. The drag only diminishes with increasing speed, so at low to medium speeds the drag can result in less-than-optimal energy efficiency.
Another EDS prototype named Inductack was invented by Dr. Post of Lawrence Livermore National Lab. (U.S. Pat. No. 6,664,880 December 2003, and U.S. Pat. No. 6,633,217 October 2003). Inductrack uses NdPM Halbach arrays in the vehicle interacting with copper wire coils. Inductrack prototypes have demonstrated a potential lift to drag ratio (L/D) of about 400:1 at 200 mph, this about five times the rolling resistance of a steel wheel high speed train at the same speed. A similar EDS-PM arrangement is disclosed in U.S. Pat. No. 7,950,333 B2 to Crawford et al (Disney) with no provisions for interchange or switching.
US 2008/0148988 A1 and U.S. Pat. No. 8,171,858 May 2012 and U.S. Pat. No. 7,562,628 all by Wamble et al (Skytran) uses NdPM elements and electric coils in a configuration allowing non-mechanical switching, however the drag force is high at low speeds, and it requires provision for touchdown at low speed or stops. The Skytran switch design has speed limitations imposed by safety and structural limitations of the converge/diverge angle (or risk of diverge failure). Further limitations of Skytran are levitation force is reduced in the zone of the diverge/converge segment necessitating double the amount of magnetic material (and/or passive lift coils) in the tube and/or vehicle. A further limitation is that in the diverge zone of the switch, active electric magnetic force (with position control feedback) is required to counter the unbalanced passive repulsion force on the continuation path side if the vehicle is desired to continue on; OR to supply active electromagnetic divergence force and electronic sensing and control for a diverge to occur. If electronic sensing and force control is not used, rollers or skids are required to prevent unwanted contact with magnetic components. Very high reserve forces in the electromagnetic elements in a diverge zone are required to counter variable side wind forces acting on the vehicle. Yet another limitation is that the steering forces do not act on the center of gravity of the vehicle, and swinging of the vehicle is likely to occur from lateral switching forces and/or passenger movements or wind force. Another problem is that propulsive forces act far from the center of gravity of the vehicle potentially causing pitching excitations that require clamping force generators to overcome.
U.S. Pat. No. 5,631,617 May 1997 to Morishita of Japan, and U.S. Pat. No. 7,197,987 B2 April 2007 Falter et al. of Germany disclose High Temperature Superconductor Maglev (HTSM) that uses NdPM that interacts in attraction and/or repulsion with diamagnetic YBCO (Yttrium-Barium-Copper-Oxide) superconductive bulk crystals in the vehicle. HTSM levitation is capable of producing attractive force, repulsive force, and shear force between the superconductive (SC) elements and the Permanent magnet (PM) elements. HTSM levitation force and restoring force is dependent on the magnetic force gradient. For HTSM to function the SC elements (for instance YBCO) must be maintained at cryogenic temperatures (below 91 Kelvin in the case of YBCO) to enter the superconductive state. HTSM was first demonstrated to carry passengers by the inventor WANG Jaisu a professor of South West Jaiotoung (transportation) University (SWJTU) in Chengdu China 31 Dec. 1999. Prof. Wang has been granted several patents in China that relate to HTSM. Wang's HTSM is very stable without computer control or energy supply. The HTSM prototype exhibited very stiff suspension in the vertical direction, and had a small degree of freedom (about 5 mm) in the lateral direction with only a few Newtons of force, and then encountered very stiff resistance requiring over 5000N displacing another 5 mm in the lateral direction. The prototype HTSM by Wang was not optimized to reduce cost or magnetic drag force. Furthermore, the HTSM prototype at SWJTU has a problem (especially during times of humid air conditions) of ice forming on the cold surface of the vehicle immediately above the suspension gap. HTSM configured to operate in the open environment is also subject to stray ferromagnetic material being attracted to attach to the permanent magnets and pose a risk to passing vehicles. NdPM is subject to corrosion problems in the open air, and NdPM is usually plaited with corrosion resistant metals to help prolong service life of NdPM elements exposed to the atmosphere. Metallic plaiting can contribute to increased drag force.
U.S. Pat. No. 6,418,857 July 2002 Okano et al. of Japan, makes use of vacuum to mitigate the frosting and magnetic attraction and corrosion problems of HTSM, however another problem remains: the use of liquid nitrogen (LN2) as a heat sink to maintain temperatures below 91 Kelvin necessary for HTSM function. The use of LN2 for ETT-HTSM is not optimal due to the large volume change of gas phase compared to liquid phase. Use of LN2 for ETT-HTSM would necessitate either onboard compression and storage (heavy, expensive, and energy intensive); or release of the N2 gas to the evacuated environment in the tubes (loading the vacuum pumps and dramatically increasing energy use). The HTSM prototype by Wang used about 35 liters of LN2 during the maximum levitation time of 7 hours. The LN2 boils away and is vented to the atmosphere. While not a poison, N2 gas can displace oxygen in enclosed spaces and result in asphyxiation. Okano provides no way to interchange vehicles.
Experience with rotary HTSM bearings has shown that the L/D can be as high as a hundred million. The HTSM prototype in Chengdu required a large quantity of expensive neodymium permanent magnets (NdPM), and used a configuration of NdPM that produced significant drag force resulting in a L/D of about 1000:1.
HTSM configurations that use High energy Neodymium PM material (NdPM) in Halbach arrays to focus the magnetic force and thereby reduce the quantity of magnetic material necessary to generate a levitation force; and do not require electrically conductive soft magnetic elements that exhibit increased eddy current drag.